“Extrapolate what we do know, and you work back to “the Big Bang” pretty easily;”
Absolutely false! Extrapolation must be based on observed phenomenon, and yet that is explicitly abandoned at a certain point in the Big Bang models. If it was not, the models would predict a collapse to a black hole, from which no matter could escape, and therefore, no universe could ever emerge from the singularity.
That would only happen if the matter formed too fast relative to the expansion of space. Obviously it didn’t. And space is, obviously, expanding.